The present invention relates to a carburetor float chamber venting system, and more particularly relates to a float chamber venting system for a carburetor of the sort which incorporates a fuel vapor absorption device such as a charcoal canister.
The float chamber of a carburetor for use in an internal combustion engine such as an engine for an automotive vehicle is generally vented at its upper portion, for the purpose of adjusting the pressure within said upper portion, so as to allow the normal and well known process of adjustment of the level of fuel within the float chamber and passing of this fuel into the intake throat or throats of the carburetor to continue without being interfered with by pressure variation in said upper portion of said float chamber. In view of the standards for prevention of emission of noxious components, and particularly of hydrocarbons such as evaporated gasoline, from internal combustion engines of motor vehicles, which are becoming more and more severe nowadays, it is usually practice for the upper part of the float chamber to be vented, in fact, to the intake throat of the carburetor, or more generally to the intake system of the engine at some point, via an intake system float chamber venting passage which leads from said upper part of the float chamber to said intake system. Thus, one end of said intake system float chamber venting passage is communicated to said upper part of the float chamber at a certain point of said upper part of said float chamber, while the other end of said intake system float chamber venting passage is communicated to said intake system of said engine.
Such an arrangement is effective for venting the upper part of the float chamber when the engine is being operated, and for disposing of the vapors that inevitably evaporate from the upper free surface of the fuel in the float chamber into said upper part thereof when the engine is being operated, but is however unsatisfactory with regard to the disposal of the vapors that also inevitably evaporate from the upper free surface of the fuel in the float chamber into said upper part thereof when the engine is not being operated, because such vapors may well pass from the intake system of the engine to which they have been conducted through said intake system float chamber venting passage from the upper part of the float chamber so as to escape to the outside atmosphere, since when the engine is not being operated of course such vapors are not sucked into the engine and combusted therein. Accordingly, it is commonly practiced to provide a device for absorbing such fuel vapors such as a charcoal canister, and to connect this fuel vapor absorption device to the upper part of the float chamber via a fuel vapor absorption device float chamber venting passage, which is provided independently of the abovementioned intake system float chamber venting passage. Thus, one end of said fuel vapor absorption device float chamber venting passage is communicated to said upper part of the float chamber at a point of said upper part of said float chamber somewhat removed from the aforesaid certain point of said upper part of said float chamber to which said one end of said intake system float chamber venting passage is communicated, while the other end of said fuel vapor absorption device float chamber venting passage is communicated to said fuel vapor absorption device.
This fuel vapor absorption device float chamber venting passage is kept closed when the engine is in operation by a valve of some sort or the like, so as to prevent the fuel vapors that inevitably evaporate from the upper free surface of the fuel in the float chamber into said upper part thereof when the engine is being operated from passing through said fuel vapor absorption device float chamber venting passage into said fuel vapor absorption device, and so as to force said vapors instead to pass through said intake system float chamber venting passage into said intake system of said engine so as to be combusted in said engine and thus disposed of; but on the other hand when the engine is stopped this fuel vapor absorption device float chamber venting passage is opened by the opening of said valve or the like, so as to allow the aforesaid fuel vapors that inevitably evaporate from the upper free surface of the fuel in the float chamber into said upper part thereof when the engine is not being operated to pass through said fuel vapor absorption device float chamber venting passage into said fuel vapor absorption device so as to be absorbed by an active absorbing substance such as activated charcoal contained therein. Thus, contamination of the atmosphere by the release of such fuel vapors, when the engine is not being operated, through the intake system float chamber venting passage into said intake system of said engine and thence to the atmosphere is positively prevented.
Such a carburetor float chamber venting system as described above is effective with regard to the prevention of contamination of the atmosphere by the release of fuel vapors when the engine is not being operated through the intake system float chamber venting passage and the intake system to the outside air, but a problem arises in its operation, as follows. When the engine is first switched off, so that the above mentioned valve or the like is opened from being closed, and so that the fuel vapor absorption device is newly communicated via the fuel vapor absorption device float chamber venting passage with the upper part of the float chamber, then, if the intake system float chamber venting passage remains open at this time, then a current of air may well be established, flowing from the intake system of the engine, through the intake system float chamber venting passage in the reverse direction to the previous flow of fuel vapors, over the surface of the fuel which is still remaining within the float chamber between said one end of said intake system float chamber venting passage and said one end of said fuel vapor absorption device float chamber venting passage, through said fuel vapor absorption device float chamber venting passage, and into the fuel vapor absorption device. This air current may be established, for instance, because of cooling of the fuel vapor absorption device and of contraction of the volume of the gas contained therein, or for other reasons. In any case, such an air current passing for any substantial length of time over the fuel remaining within the float chamber will have the very undesirable effect of preferentially evaporating the more volatile components of said fuel which is still remaining within the float chamber, so that the relative amount of less volatile components, or high boiling point components, in said fuel remaining within the float chamber is increased. Thus, the overall boiling point of the fuel remaining within the float chamber is increased, and this deteriorates starting of the engine. Further, this evaporation of said more volatile components of the fuel which is still remaining within the float chamber is rather wasteful of fuel, which is undesirable from the point of view of energy saving.
As a system to obviate this problem, there has been proposed a system in which through the use of another valve or the like, or through the use of a combination valve system, at or approximately at the time when the fuel vapor absorption device float chamber venting passage is opened, i.e. at or approximately at the time when the engine is stopped from operating, the intake system float chamber venting passage is closed. Such a system is reasonably straightforward if the fact that it requires another control valve and makes the float chamber venting system to that extent more complicated is disregarded. Further, in the case that only one such intake system float chamber venting passage is provided. However, nowadays many vehicles are equipped with so called duplex type carburetors which have both a primary fuel supply system and a secondary fuel supply system, and in such a carburetor there are typically provided two of such intake system float chamber venting passages, one leading from the upper part of the float chamber to the primary throat of the carburetor in the primary fuel supply system, and one leading separately from the upper part of the float chamber to the secondary throat of the carburetor in the secondary fuel supply system. In such a duplex carburetor, if the above mentioned solution to the problem of the establishment of an air current over the surface of the fuel remaining in the float chamber after the engine has been stopped, of controlling the intake system float chamber venting passage in inverse or opposite relation to the control of the fuel vapor absorption device float chamber venting passage, is adopted, then both of these two intake system float chamber venting passages have to be closed together as the fuel vapor absorption device float chamber venting passage is opened, and vice versa. This is more troublesome to do with only one control valve, and leads to great complication in the construction of the carburetor.